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Selasa, 03 April 2012

Method Detection Limit

DefinitionThe method detection limit (MDL) is defined as the minimum concentration of a substance thatcan be measured and reported with 99% confidence that the analyte concentration is greater thanzero and is determined from analysis of a sample in a given matrix containing the analyte.Scope and ApplicationThis procedure is designed for applicability to a wide variety of sample types ranging fromreagent (blank) water containing analyte to wastewater containing analyte. The MDL for ananalytical procedure may vary as a function of sample type. The procedure requires a complete,specific, and well defined analytical method. It is essential that all sample processing steps of theanalytical method be included in the determination of the method detection limit.The MDL obtained by this procedure is used to judge the significance of a single measurementof a future sample.The MDL procedure was designed for applicability to a broad variety of physical and chemicalmethods. To accomplish this, the procedure was made device- or instrument-independent.Procedure1. Make an estimate of the detection limit using one of the following:(a) The concentration value that corresponds to an instrument signal/noise in the range of 2.5to 5.(b) The concentration equivalent of three times the standard deviation of replicate instrumentalmeasurements of the analyte in reagent water.(c) That region of the standard curve where there is a significant change in sensitivity, i.e., abreak in the slope of the standard curve.(d) Instrumental limitations.It is recognized that the experience of the analyst is important to this process. However, theanalyst must include the above considerations in the initial estimate of the detection limit.2. Prepare reagent (blank) water that is as free of analyte as possible. Reagent or interferencefree water is defined as a water sample in which analyte and interferent concentrations are notdetected at the method detection limit of each analyte of interest. Interferences are defined assystematic errors in the measured analytical signal of an established procedure caused by thepresence of interfering species (interferent). The interferent concentration is presupposed to benormally distributed in representative samples of a given matrix.3. (a) If the MDL is to be determined in reagent (blank) water, prepare a laboratory standard(analyte in reagent water) at a concentration which is at least equal to or in the sameconcentration range as the estimated method detection limit. (Recommend between 1 and 5 timesthe estimated method detection limit.) Proceed to Step 4.(b) If the MDL is to be determined in another sample matrix, analyze the sample. If themeasured level of the analyte is in the recommended range of one to five times the estimateddetection limit, proceed to Step 4.If the measured level of analyte is less than the estimated detection limit, add a known amountof analyte to bring the level of analyte between one and five times the estimated detection limit.If the measured level of analyte is greater than five times the estimated detection limit, thereare two options.(1) Obtain another sample with a lower level of analyte in the same matrix if possible.(2) The sample may be used as is for determining the method detection limit if the analyte leveldoes not exceed 10 times the MDL of the analyte in reagent water. The variance of the analyticalmethod changes as the analyte concentration increases from the MDL, hence the MDLdetermined under these circumstances may not truly reflect method variance at lower analyteconcentrations.4. (a) Take a minimum of seven aliquots of the sample to be used to calculate the methoddetection limit and process each through the entire analytical method. Make all computationsaccording to the defined method with final results in the method reporting units. If a blankmeasurement is required to calculate the measured level of analyte, obtain a separate blankmeasurement for each sample aliquot analyzed. The average blank measurement is subtractedfrom the respective sample measurements.(b) It may be economically and technically desirable to evaluate the estimated methoddetection limit before proceeding with 4a. This will: (1) Prevent repeating this entire procedurewhen the costs of analyses are high and (2) insure that the procedure is being conducted at thecorrect concentration. It is quite possible that an inflated MDL will be calculated from dataobtained at many times the real MDL even though the level of analyte is less than five times thecalculated method detection limit. To insure that the estimate of the method detection limit isa good estimate, it is necessary to determine that a lower concentration of analyte will not resultin a significantly lower method detection limit. Take two aliquots of the sample to be used tocalculate the method detection limit and process each through the entire method, including blankmeasurements as described above in 4a. Evaluate these data:(1) If these measurements indicate the sample is in desirable range for determination of theMDL, take five additional aliquots and proceed. Use all seven measurements for calculation ofthe MDL.(2) If these measurements indicate the sample is not in correct range, reestimate the MDL,obtain new sample as in 3 and repeat either 4a or 4b.5. Calculate the variance (S2) and standard deviation (S) of the replicate measurements, asfollows:

Where:Xi; I=1 to n, are the analytical results in the final method reporting units obtained from the nsample aliquots and S refers to the sum of the X values from I=l to n.6. (a) Compute the MDL as follows:

MDL = t(n-1,1-µ = 0.99) (S)where:MDL = the method detection limitt(n-1,1-a = .99) = the students t value appropriate for a 99% confidence level and a standarddeviation estimate with n-1 degrees of freedom. See Table.S = standard deviation of the replicate analyses.(b) The 95% confidence interval estimates for the MDL derived in 6a are computed accordingto the following equations derived from percentiles of the chi square over degrees of freedomdistribution (x /df).2LCL = 0.64 MDLUCL = 2.20 MDLwhere: LCL and UCL are the lower and upper 95% confidence limits respectively based onseven aliquots.7. Optional iterative procedure to verify the reasonableness of the estimate of the MDL andsubsequent MDL determinations.(a) If this is the initial attempt to compute MDL based on the estimate of MDL formulated inStep 1, take the MDL as calculated in Step 6, spike the matrix at this calculated MDL andproceed through the procedure starting with Step 4.(b) If this is the second or later iteration of the MDL calculation, use S2 from the current MDLcalculation and S2 from the previous MDL calculation to compute the F-ratio. The F-ratio iscalculated by substituting the larger S2 into the numerator S2 and the other into the denominatorAS2 The computed F-ratio is then compared with the F-ratio found in the table which is 3.05 asBfollows: if S2 /S2 >3.05, then compute the pooled standard deviation by the following equation:A B if S2 /S2 >3.05, respike at the most recent calculated MDL and process the samples through theA Bprocedure starting with Step 4. If the most recent calculated MDL does not permit qualitativeidentification when samples are spiked at that level, report the MDL as a concentration betweenthe current and previous MDL which permits qualitative identification.(c) Use the Spooled as calculated in 7b to compute the final MDL according to the followingequation:MDL=2.681 (Spooled)where 2.681 is equal to t(12, 1-a =.99).(d) The 95% confidence limits for MDL derived in 7c are computed according to the followingequations derived from percentiles of the chi squared over degrees of freedom distribution.LCL=0.72 MDLUCL=1.65 MDLwhere LCL and UCL are the lower and upper 95% confidence limits respectively based on 14 aliquot.

Reporting

The analytical method used must be specifically identified by number or title and the MDL foreach analyte expressed in the appropriate method reporting units. If the analytical methodpermits options which affect the method detection limit, these conditions must be specified withthe MDL value. The sample matrix used to determine the MDL must also be identified withMDL value. Report the mean analyte level with the MDL and indicate if the MDL procedure wasiterated. If a laboratory standard or a sample that contained a known amount analyte was usedfor this determination, also report the mean recovery.If the level of analyte in the sample was below the determined MDL or exceeds 10 timesthe MDL of the analyte in reagent water, do not report a value for the MDL.

APPENDIX B TO PART 136 - DEFINITION AND PROCEDURE FOR THE DETERMINATION OF THEMETHOD DETECTION LIMIT - REVISION 1.11